WO2015110753A1

WO2015110753A1 - Cooling method comprising a cooling facility, and cooling facility

Info

Publication number: WO2015110753A1
Application number: PCT/FR2015/050140
Authority: WO
Inventors: Bruno Berthome; Fabrice Rivoal; Etienne Werlen; Louis DE BESOMBES
Original assignee: L'air Liquide, Societe Anonyme Pour L'etude Et L'exploitation Des Procedes Georges Claude
Priority date: 2014-01-21
Filing date: 2015-01-20
Publication date: 2015-07-30
Also published as: FR3016687B1; FR3016687A1

Abstract

The invention relates to a cooling method implementing a cooling facility (2) comprising at least one exchanger (6) for transferring heat from the heat-producing fluid to the refrigerant fluid, the cooling facility (2) being connected to a compressor (4) for compressing refrigerant fluid. Said cooling method comprises a so-called dry vapourisation step wherein the exchanger (6) is supplied with heat-producing fluid and liquid refrigerant fluid, and wherein all of the liquid refrigerant fluid flowing into the exchanger (6) is vapourised. Said cooling method also comprises a so-called supercharging step wherein the flow of liquid refrigerant fluid entering the exchanger (6) is determined such that the refrigerant fluid leaving the exchanger (6) comprises liquid refrigerant fluid.

Description

COOLING PROCESS COMPRISING AN INSTALLATION OF

COOLING AND COOLING INSTALLATION

The present invention relates to a cooling method, implementing a cooling system, a circulating fluid by a refrigerant. In addition, the present invention relates to an installation for cooling a circulating fluid with a refrigerant.

The present invention can be applied to any industrial field requiring cooling of a circulating fluid by a refrigerant. In particular, the present invention can be applied to the condensation of gaseous carbon dioxide by heat exchange with ammonia.

In a conventional cooling process and plant, refrigerant is vaporized in a heat exchanger operating as a thermosiphon. The vapors of the refrigerant are then compressed by a compressor, then liquefied at room temperature before being expanded and redirected to the exchanger operating thermosiphon. However, such a cooling installation is very bulky and it requires a large amount of refrigerant, because it is necessary to ensure the two-phase separation of the fluid exiting the exchanger and the recirculation of the liquid in the exchanger. Due to the cost of the refrigerant, the cost of the installations as well as the applicable regulations, it is generally sought to limit the amount of refrigerant in the cooling installation.

Hegerland et al, International Conference on Greenhouse Gas Control Technologies, 2004, describes a "kettle" type heat exchanger. In this case, the refrigerant is not vaporized to dry insofar as the tubes for vaporizing the refrigerant are embedded. There is therefore no risk of oil deposit on the walls of the exchanger tubes since the oil remains in the liquid phase. A purge at low point of the exchanger generally helps to deconcentrate the oil. The disadvantage of this exchanger is its large volume, requiring a large amount of refrigerant and a high cost. It does not therefore solve all the disadvantages that our system proposes to solve (including a small amount of refrigerant in the exchanger). To reduce the amount of liquid refrigerant, one solution is to use a process and a cooling system, vaporizing most or all (so-called "dry vaporization") of the refrigerant in the exchanger, instead of establish a thermosiphon. The cooling system comprises a compressor for compressing the vapors of the refrigerant leaving the exchanger and then liquefying them at ambient temperature.

This compressor is often of lubricated type. As a result, at the compressor outlet, part of the oil used for the lubrication is driven by the vapor phase of the refrigerant in the form of vesicles. A coalescing filter makes it possible to eliminate most of the oil entrained, but a small part of the oil manages to cross the filter in the liquid or vapor state. Then, after the refrigerant has condensed, the oil is transported by the liquid phase into the cooling plant.

Usually, as described in EP-A-049081 0, to limit the formation of oil deposition, an oil miscible in the refrigerant is used, this miscible oil being for example of polyalkylene glycol (PAG) type with a limit of solubility in the refrigerant of 1% or more at the temperature of use, for example between -20 ° C and -40 ° C. Such a miscible oil generally, and particularly in ammonia, has a high low temperature viscosity which limits the cooling process to temperatures generally above -20 ° C.

Indeed, one of the disadvantages associated with the use of this type of oil is that it is only partially driven by the liquid refrigerant during the dry vaporization step, and accumulates gradually over the walls of the exchanger. Thus, the gaseous refrigerant can not evacuate this oil out of the exchanger because it is very viscous at low temperature. The deposited oil layer reduces the heat transfer and the flow section, which decreases the performance of the exchanger and increases the pressure drops. EP-A-049081 0 therefore recommends that the refrigerant and the vaporised refrigerant liquid be taken out separately from the exchanger.

The present invention aims in particular to solve, totally or partially, the problems mentioned above. For this purpose, the subject of the invention is a cooling method, for cooling a heat transfer fluid by heat exchange with a refrigerant, for example for condensing gaseous carbon dioxide by heat exchange with ammonia, the cooling method implementing a cooling installation comprising at least one exchanger adapted to transfer heat from the circulating fluid to the refrigerant, the cooling installation comprising a compressor lubricated and adapted to compress refrigerant,

The cooling method comprising a so-called dry vaporization step including actions of: i) supplying the exchanger with circulating fluid and liquid refrigerant, and ii) vaporizing all the liquid refrigerant flowing in the exchanger,

The cooling method being characterized in that it further comprises a so-called supercharging step which takes place after the dry vaporization step (101) and which includes actions consisting of: i) feeding the exchanger (6) with circulating fluid and liquid refrigerant, and ii) adjust the flow rate of the liquid refrigerant entering the exchanger so that the refrigerant leaving the exchanger comprises liquid refrigerant.

Thus, such a cooling method avoids the accumulation of oil in the active part of the exchanger. Indeed, during a supercharging step, the oil accumulated in the exchanger during the dry vaporization step is effectively removed. The oil is thus re-entrained and then discharged by the liquid refrigerant leaving the exchanger, the supercharging stage allowing somehow a rinsing of the exchanger by detaching the oil deposited on its inner walls. The supercharging step and the dry vaporization step are not simultaneous. The supercharging step follows the dry vaporization step. In other words, the dry vaporization step constitutes a first mode of operation of the cooling installation and the supercharging stage constitutes a second mode of operation of the cooling installation, this second mode of operation intervening after the first mode of operation.

Therefore, such a cooling process can be carried out at low temperatures, i.e. at temperatures below the temperatures allowable minimum for a prior art cooling process, typically below -20 ° C.

In the present application, the term "exchanger" designates a heat exchanger, that is to say a heat exchanger. In the exchanger, while the heat transfer fluid transfers calories to the refrigerant, the refrigerant transfers frigories to the circulating fluid. For this purpose, the exchanger has at least: a refrigerant inlet, a refrigerant outlet, a circulating fluid inlet and a circulating fluid outlet.

In the present application, the verbs "connect", "connect", "feed" and their derivatives relate to the communication fluid of at least two volumes distant from each other, to allow a flow of fluid between these two volumes apart. This fluid flow can be via any one or more component (s), that is, directly or indirectly.

According to a variant of the invention, the dry vaporization step has a first predetermined duration and the supercharging step has a second predetermined duration, the ratio of the second predetermined duration to the first predetermined duration being less than 10%.

In practice, to vaporize all the refrigerant, it is necessary to provide a quantity of heat that exceeds the enthalpy of vaporization of the refrigerant entering the exchanger. For this purpose, we can:

regulate the temperature of the refrigerant leaving the exchanger, for example by heating the outgoing refrigerant, and / or

regulating the flows of the refrigerant and the heat transfer fluid entering the exchanger, so that the amount of heat supplied by the heat transfer fluid exceeds the evaporation enthalpy (latent heat) for the amount of refrigerant entering the exchanger.

According to one embodiment of the invention, the cooling method further comprises a cycle step of cyclically repeating the dry vaporization step and the supercharging step, the dry vaporization step having a first duration predetermined and the supercharging step having a second predetermined duration, a ratio of the second predetermined duration to the first predetermined duration being preferably less than 10%. Thus, such a threshold for this ratio makes it possible to optimize the efficiency of the cooling process, since the accumulation of oil in the exchanger is minimized during the second period, and the vaporization of refrigerant during the first period is maximized. duration, which is substantially shorter than the second duration. The ratio is selected according to the dimensions of the cooling system, in particular the type of exchanger, the flow rate of circulating coolant, etc.

According to a variant of the invention, the supercharging step preferably succeeds directly to the dry vaporization step. Thus, it optimizes the rinsing time of the exchanger, to evacuate the accumulated oil.

Preferably, the compressor contains an immiscible oil in liquid ammonia (liquid NH ₃ ). This oil may be, for example, a synthetic oil of the polyalphaolefin (PAO) type.

According to one embodiment of the invention, the cooling installation further comprises:

a collection tank arranged to contain liquid refrigerant leaving the exchanger and the gaseous refrigerant leaving the exchanger,

a first sensor, said low level sensor, adapted to emit a first signal when the volume of liquid refrigerant contained in the collection tank reaches a predetermined lower threshold,

a second sensor, called high level sensor, adapted to emit a second signal when the volume of liquid refrigerant contained in the collection tank reaches a predetermined upper threshold, and

a control unit configured to control the cooling installation so as to carry out:

a first transition from the dry vaporization stage to the supercharging stage when the low level sensor emits the first signal, then

a second transition from the supercharging step to the dry vaporization step when the high level sensor outputs the second signal.

Thus, the first transition makes it possible, thanks to the low level sensor, to begin flushing the exchanger in order to evacuate the oil that has accumulated there during the dry vaporization step. The second transition allows, thanks to the high level sensor, avoid accumulation of non-consumable liquid refrigerant in the collection tank.

In the present application, the term "high" qualifies an element located at a higher altitude than a "low" element when the cooling installation is in service. The high level may for example be set at about 40% of the volume of the collection tank, while the low level may for example be set at about 10% of the volume of the collection tank.

Preferably, the collection tank receives a mixture of vaporized and non-vaporized refrigerant during the supercharging step. Thus by vaporizing the vaporized liquid entrains the non-vaporized liquid. The vaporized refrigerant can be sent directly from the collection tank to a refrigerating cycle compressor.

Preferably, the collection tank does not receive non-vaporized refrigerant during the dry vaporization step.

According to one embodiment of the invention, the control unit is configured to control the cooling installation so as to perform the first transition so that the flow rate of the liquid refrigerant entering the exchanger increases gradually, then way to achieve the second transition so that the flow of the liquid refrigerant entering the exchanger gradually decreases.

Thus, such gradual transitions have a limited impact on the overall performance of the cooling plant.

a feed tank adapted to contain liquid refrigerant under a so-called mean pressure, for example 5 barA,

a separator pot adapted to separate the liquid refrigerant from the gaseous refrigerant under a so-called low pressure, for example of 1.2 barA, which is lower than the average pressure, the separator pot being arranged between the supply tank and the exchanger so as to receive refrigerant from the supply tank and to transmit liquid refrigerant preferably by gravity to the exchanger, an expansion valve arranged between the supply tank and an inlet of the exchanger so as to relax the liquid refrigerant from the mean pressure to the low pressure,

a collection tank arranged to contain liquid refrigerant leaving the exchanger and the gaseous refrigerant leaving the exchanger, so as to recover the oil entrained at the outlet of the exchanger and to separate any excess refrigerant liquid,

The cooling method further comprises a control step in which the flow section of the expansion valve is regulated so as to condense the bulk of the flow of available heat transfer fluid, in particular by regulating a caloric fluid pressure at the inlet of the exchanger, and thus to control the flow of liquid refrigerant entering the exchanger.

Thus, it is possible to recover the oil entrained at the outlet of the exchanger and to separate any excess liquid refrigerant. The separator pot processes a low vapor flow, equal to the fraction vaporized by free expansion of the medium pressure at low pressure (process called "flash"). The separator pot may be compact and in particular have a smaller volume than the feed tank and the collection tank. This compactness of the separator pot makes it possible to place the separator pot, therefore the exchanger in height, typically above a distillation column. In certain configurations, placing the exchanger in height avoids the installation of a pump for supplying the circulating fluid at the head of a distillation column.

According to an alternative to this variant, the cooling process is performed without separator pot, which simplifies the cooling installation. In this case, the exchanger is fed with a two-phase refrigerant, which can however reduce the performance of the exchanger because of poor distribution of the refrigerant in the exchanger.

In a cooling process of the prior art, the container filling the function of separator pot had to be placed at high height, which made access difficult. In addition, in a cooling process of the prior art, the same container had to have a very large volume, which represented risks to the safety of people or to the integrity of the equipment, because some refrigerants and / or calorigenic produce an explosive atmosphere. Alternatively, in another art cooling process previous, the heat exchanger was placed on the ground and it was necessary to provide a pump to return the liquid heat transfer fluid at the top of the distillation column.

According to a variant of the invention, the cooling installation further comprises a bypass line connecting the separator pot directly to the outlet of the exchanger, the cooling method comprising a step of deriving gaseous refrigerant from the pot separator towards the exit of the exchanger. Thus, the bypass line makes it possible to return the vaporized fraction by free expansion from the medium pressure to the low pressure (so-called "flash" process) towards the suction of the compressor.

According to one embodiment of the invention, the collection tank operates a two-phase separation of the refrigerant so as to separate the liquid refrigerant from the gaseous refrigerant, the cooling process further comprising a channeling step in which the refrigerant gas is channeled from the collection tank to the compressor.

Thus, such a phase separation of the refrigerant makes it possible to recover the gaseous refrigerant and then recondense it after compression before redirecting it to the exchanger.

According to one embodiment of the invention, the cooling installation further comprises regulating means for transferring heat to the refrigerant so as to superheat the refrigerant leaving the exchanger, preferably by regulating the flow rate. of the refrigerant and the flow rate of the circulating fluid so that the temperature of the refrigerant leaving the exchanger is at least 3 ° C higher than the dew point temperature of the refrigerant, calculated at the operating pressure, for example of 1, 2 barA.

In other words, the refrigerant is superheated in the exchanger or at its outlet. Thus, such regulating means make it possible to supply the refrigerant with a quantity of heat which exceeds the enthalpy of vaporization for the quantity of refrigerant entering the exchanger, which makes it possible to carry out the dry vaporization step.

Alternatively or concomitantly with the superheating by the regulating means, it is possible to regulate the flow rates of the refrigerant and the heat transfer fluid entering the exchanger, so that the quantity of heat supplied by the circulating fluid exceeds the evaporation enthalpy (heat latent) for the amount of refrigerant entering the exchanger, so that the fluid temperature refrigerant leaving the exchanger is at least 3 ° C higher than the dew point temperature of said refrigerant, calculated at operating pressure, for example 1 .2 barA.

In practice to achieve this superheating, the cooling process may further comprise a control step in which the flow section of the expansion valve is regulated so as to ensure the overheating of the refrigerant at the outlet of the exchanger, in particular by regulating the temperature difference of the refrigerant leaving the exchanger with the dew point temperature of said refrigerant.

The difference of at least 3 ° C between the temperature of the refrigerant leaving the exchanger and the temperature of the two-phase equilibrium makes it possible to guarantee the vaporization of all the liquid refrigerant while avoiding the uncertainties of measurement.

Any refrigerant vaporized in the exchanger, during the dry vaporization and possibly during the supercharging, is sent from the exchanger to the compressor through the collection tank.

According to one embodiment of the invention, the refrigerant is selected from the group consisting of ammonia, freon, ethane, propane and any mixture of said refrigerants, and the circulating fluid is selected in the group consisting of carbon dioxide, nitrous oxide, hydrocarbons, in particular hydrocarbons whose molecule comprises two to four carbon atoms, or any mixture of said calorogenic fluids, with possibly impurities such as nitrogen, oxygen, hydrogen, methane and / or carbon monoxide.

Thus, such refrigerant and calorigenic fluids allow to obtain vapors and condensates specific to certain applications.

According to a variant of the invention, the cooling method implements concomitantly several refrigerants and / or several heat transfer fluids.

According to one embodiment of the invention, the refrigerant is ammonia, the circulating fluid being, for example, pure or impure carbon dioxide, and the temperature of the refrigerant leaving the exchanger is between -20.degree. C and -50 ° C, for example equal to about -30 ° C. Thus, ammonia makes it possible to carry out a cooling process at low temperature. The temperature of -26 ° C. is 4 ° C. higher than the temperature of the two-phase equilibrium of ammonia which is -30 ° C. under a pressure of 1.2 barA.

Thus, the cooling plant can operate in dry vaporization at lower temperatures than a prior art cooling plant, while having the advantages of compactness and low refrigerant amount.

According to one embodiment of the invention, during the supercharging step, the flow rate of the liquid refrigerant entering the exchanger is determined so that the refrigerant leaving the exchanger comprises a mass of liquid refrigerant representing at least 10% of the mass of gaseous refrigerant.

In other words, it ensures an excess of liquid preferably representing between 10% and 100% of the mass of vaporized refrigerant. For example, for 2 kg of liquid ammonia entering, an excess of 100% represents 1 kg of liquid ammonia leaving and 1 kg of ammonia gas leaving, while a 10% excess represents 0.2 kg of ammonia outgoing liquid and 1.8 kg of ammonia gas exiting.

Thus, such an excess of liquid refrigerant allows a particularly effective removal of the oil from the exchanger.

Moreover, the subject of the present invention is a cooling installation, intended to cool a heat transfer fluid by heat exchange with a refrigerant, for example to condense gaseous carbon dioxide by heat exchange with ammonia, the installation of cooling device comprising at least one exchanger adapted to transfer heat from the circulating fluid to the refrigerant, the cooling installation comprising a compressor adapted for compressing refrigerant.

This cooling system is characterized in that it further comprises a control unit configured to operate the following cooling system:

a so-called dry vaporization step including actions of: i) supplying the exchanger with circulating fluid and liquid refrigerant, and ii) vaporizing all the refrigerant flowing in the exchanger, and a so-called supercharging step which takes place after the so-called dry vaporization step and which includes actions consisting in: i) supplying the exchanger (6) with circulating fluid and liquid refrigerant, and ii) adjusting the flow rate of the fluid liquid refrigerant entering the exchanger so that the refrigerant leaving the exchanger is composed wholly or partially of liquid refrigerant.

Thus, such a cooling installation avoids the accumulation of oil in the active part of the exchanger. Indeed, during a supercharging step, the oil accumulated in the exchanger during the dry vaporization steps can be evacuated effectively. The supercharging stage allows somehow a rinsing of the exchanger by detaching the oil deposited on its inner walls. Thus, such a cooling installation does not require a system for recirculation of the excess liquid at the outlet of the exchanger, whether by pump or by thermosiphon.

Therefore, such a cooling plant can be operated at low temperatures, i.e. at temperatures below the minimum allowable temperatures for a prior art cooling plant, typically below -20. ° C.

According to one embodiment of the invention, the exchanger is a plate exchanger, preferably brazed aluminum.

Thus, such an exchanger can effectively handle large flow rates of refrigerant and heat sink and can provide a minimum refrigerant speed for driving the oil, while having a footprint, weight and reduced cost.

a first sensor, said low level sensor, adapted to emit a first signal when the volume of liquid refrigerant contained in the collection tank reaches a predetermined lower threshold, and a second sensor, said high level sensor, adapted to emit a second signal when the volume of liquid refrigerant contained in the collection tank reaches a predetermined upper threshold.

Thus, the low level sensor makes it possible to begin flushing the exchanger in order to evacuate the oil that has accumulated there during the dry vaporization step. The high level sensor prevents accumulation of non-consumable liquid refrigerant. The high and low level sensors make it possible to cyclically operate a cooling method according to the invention.

According to a variant of the invention, the high level sensor and the low level sensor are selected from the group comprising a capacitive sensor and an inductive sensor, the high level sensor and the low level sensor being arranged on a wall of the device. collection tank so as to detect the presence of the liquid refrigerant respectively at a high altitude and at a low altitude. Thus, such low level and high level sensors durably allow precise detections of the high and low altitudes.

According to a variant of the invention, the collection tank comprises a discharge orifice of the oil decanted by gravity in the collection tank. Thus, the oil discharged from the exchanger can be collected. This drain hole is located in the bottom of the collection tank.

According to one embodiment of the invention, the cooling installation further comprises a secondary heat exchanger fluidically connected to the heat exchanger and the collection tank so as to transfer heat from the heat transfer fluid from the exchanger to the refrigerant. liquid from the collection tank.

Thus, the secondary heat exchanger, usually called "subcooler" allows to "consume" liquid refrigerant by contributing to the vaporize. The subcooler can operate according to the thermosiphon principle.

a separator pot adapted to separate the liquid refrigerant from the gaseous refrigerant under a so-called low pressure, for example 1, 2 barA, which is lower than the average pressure, the separator pot connecting the feed tank to the exchanger so as to receive refrigerant from the supply tank and to supply the exchanger liquid refrigerant preferably by gravity, the volume the separator pot being smaller than the volume of the feed tank, and

an expansion valve arranged between the supply tank and an inlet of the exchanger so as to relax the liquid refrigerant from the mean pressure to the low pressure and thus to control the flow of liquid refrigerant entering the exchanger. The installation does not include any means for sending vaporized refrigerant into the exchanger of the exchanger to the compressor without passing through the collection tank.

In practice, it is possible to regulate, by means of the expansion valve, the flow entering the exchanger so as to condense the bulk of the flow of available heat-exchange fluid, in particular by regulating a caloric fluid pressure at the inlet or the outlet of the heat exchanger. exchanger. Advantageously, the regulation of the flow section of the expansion valve can be carried out by a control member, in particular according to the dimensions of the cooling system, in particular the exchanger, and the flows of the circulating and refrigerant fluids.

Thus, it is possible to recover the oil entrained at the outlet of the exchanger and to separate any excess liquid refrigerant. The separator pot processes a low vapor flow, equal to the fraction vaporized by free expansion of the medium pressure at low pressure (process called "flash"). The compactness of the separator pot makes it possible to place the separator pot, therefore the exchanger in height, typically above a distillation column. In certain configurations, placing the exchanger in height also makes it possible to circulate the circulating fluid without a pump.

According to an alternative to this variant, the cooling process is performed without separator pot, which simplifies the cooling installation. In this case, the exchanger is supplied with a two-phase refrigerant, which can however reduce the performance of the exchanger.

According to one embodiment of the invention, the cooling installation further comprises a branch line connecting the separator pot directly to the outlet of the exchanger. Thus, the bypass line makes it possible to return the vaporized fraction by free expansion from the medium pressure to the low pressure (so-called "flash" process) towards the suction of the compressor.

In a symmetrical manner, the subject of the invention is a vaporization method, for vaporising a refrigerant at low temperature by heat exchange with a heat-generating fluid, for example for vaporizing ammonia by thermal exchange with carbon dioxide, the process spraying device implementing a vaporization installation comprising at least one heat exchanger adapted to transfer heat from the circulating fluid to the refrigerant, the vaporization installation being connected to a compressor adapted for compressing refrigerant,

the vaporization process comprising a so-called dry vaporization stage, in which the heat exchanger is supplied with circulating fluid and liquid refrigerant, and in which all the liquid refrigerant flowing in the exchanger is vaporized,

the vaporization process being characterized in that it further comprises a so-called supercharging step in which the flow rate of the liquid refrigerant entering the exchanger is determined so that the refrigerant leaving the exchanger comprises liquid refrigerant .

The embodiments and variants mentioned above may be taken individually or in any technically permissible combination.

The present invention will be well understood and its advantages will also emerge in the light of the description which follows, given solely by way of nonlimiting example and with reference to the appended drawings, in which:

- Figure 1 is a schematic view of a cooling system according to the invention, during a first step of a cooling method according to the invention;

Figure 2 is a schematic view of the installation of Figure 1, during a second step of the cooling method according to the invention;

FIG. 3 is a logic diagram schematically illustrating a cooling method according to the invention; and

Figure 4 is a view similar to Figure 1 of a cooling system according to a variant of the invention. Figures 1 and 2 illustrate a cooling system 1 which is intended to cool carbon dioxide (CO ₂ ) as a heat transfer fluid, by heat exchange with ammonia (NH ₃ ), as a refrigerant. The cooling system 1 comprises a cooling system 2 and a compressor 4 adapted to compress refrigerant.

To facilitate the reading of FIGS. 1 and 2, the flows of ammonia in the cooling installation 2 are represented:

by strong broken lines when the ammonia only flows in the vapor phase (NH ₃ vapor, reference 15), and

- by strong continuous lines when the ammonia flows only in the liquid phase (liquid NH ₃ , reference 17) or in liquid phase and vapor phase mixture (NH ₃ liquid + vapor).

Moreover, in each tank illustrated in FIGS. 1 and 2, only liquid ammonia (liquid NH ₃ ) is represented, not steam ammonia (NH ₃ vapor); however, steam ammonia (NH ₃ vapor) is present in each tank illustrated in Figures 1 and 2 above the liquid ammonia (liquid NH ₃ ).

In addition, the cooling system 1 comprises a condenser 5 which is disposed downstream of the compressor 4 and which is adapted to condense the refrigerant. In the example of Figure 1, the condenser 5 is an evaporative condenser.

The compressor 4 is here screw compressor type lubricated. The compressor 4 cooperates with an evaporative condenser so as to liquefy steam ammonia (NH ₃ vapor) at atmospheric temperature, with a high-pressure receiver 19 (here at 12 barA) and with an expansion step to bring the liquid ammonia (liquid NH3) at medium pressure (here 5 barA).

The lubricated compressor 4 contains an immiscible oil in liquid ammonia (liquid NH ₃ ). This oil may be, for example, a synthetic oil of the polyalphaolefin (PAO) type. This high kinematic viscosity allows this oil to form with the refrigerant an emulsion light enough to allow its transport.

In the cooling system 1, the cooling system 2 is connected to the compressor 4. The cooling system 2 is intended to condense in liquid form (liquid CO ₂ , reference 13) carbon dioxide gas (CO ₂ gas) by heat exchange with ammonia (NH ₃ ). For this purpose, the cooling system 2 comprises an exchanger 6 which is adapted to transfer heat from the gaseous carbon dioxide (CO ₂ gas) to the ammonia (NH ₃ ). The exchanger 6 is here a plate and fin heat exchanger formed of brazed aluminum.

The cooling installation 2 further comprises a control unit 8, which is here an electronic control unit, configured to operate the cooling installation 2 according to a cooling method according to the invention and detailed below.

The cooling system 2 further comprises a collection tank 10 which is arranged to contain liquid ammonia (liquid NH ₃ ) and steam ammonia (NH ₃ vapor) which exit the exchanger 6, so that recovering the oil entrained at the outlet of the exchanger 6 and separating any excess liquid refrigerant. The cooling installation 2 comprises an outlet duct 11 which connects the exchanger 6 to the collection tank 10, so as to channel the ammonia leaving the exchanger 6. In the example of the figures, the collection tank 10 has a total volume of about 2000 L. In use, the collection tank 10 operates under a so-called low pressure, which is between 1 barA and 1.5 barA, for example 1, 2 barA.

The cooling installation 2 further comprises:

a first sensor, called low level sensor 12, which is adapted to emit a first signal when the volume of liquid ammonia (liquid NH ₃ ) contained in the collection tank 10 reaches a predetermined lower threshold 10.12, and

a second sensor, said high level sensor 14, adapted to emit a second signal when the volume of liquid ammonia (NH ₃ liquid) contained in the collection tank 10 reaches a predetermined upper threshold 10.14.

The low level sensor 1 2 and the high level sensor 14 are here each formed by a capacitive sensor. The low level sensor 12 and the high level sensor 14 are arranged on a wall of the collection tank 10 so as to detect the presence of liquid ammonia (liquid NH 3) respectively at a low altitude and at a high altitude, relative to at the bottom of the collection tank 10. The low altitude and the high altitude respectively define the predetermined lower threshold 10.12 and the predetermined upper threshold 10.14. The collection tank 10 comprises a drain port 1 6 for draining the oil settled by gravity in the collection tank 10. In use, the oil in the collection tank 10 decants, and the control unit 8 can regularly open a valve closing the drain port 16, so as to drain all or part of the oil.

In addition, the cooling installation 2 comprises:

a feed tank 18 adapted to contain liquid ammonia (liquid NH3) under a so-called mean pressure, for example 5 barA,

a separator pot 20 adapted to separate the liquid ammonia (liquid NH 3) from the steam ammonia (NH 3 vapor) under a so-called low pressure, for example from

1, 2 barA, which is below the average pressure, and

an expansion valve 22 arranged between the feed tank 18 and the inlet of the exchanger 6, here upstream of the separator pot 20, so as to relax the liquid ammonia (liquid NH 3) from the mean pressure (e.g. 5 barA) at low pressure (eg 1, 2 barA),

- A storage tank 19 adapted to contain liquid ammonia (liquid NH3) under a so-called high pressure, for example 1 2 barA, the storage tank 19 is usually called "high pressure receiver"; the storage tank 19 is fluidly connected to the feed tank 18;

a valve 21 arranged between the feed tank 1 8 and the storage tank 19 to regulate the flow of refrigerant.

The separator pot 20 connects the feed tank 18 to the exchanger 6 so as to receive ammonia from the feed tank 18 and to supply the exchanger 6 with liquid ammonia. The separator pot 20 supplies the exchanger 6 by gravity.

The volume of the separator pot 20 is substantially smaller than the volume of the collection tank 10. In the example of FIGS. 1 and 2, the separator pot 20 has a volume of the order of 25 times smaller than the volume of the collection tank 10. The separator pot 20 and the exchanger 6 can be placed above a distillation column, at a height of 15 m, which dispenses here to circulate the carbon dioxide with a pump.

The expansion valve 22 is regulated so as to condense the bulk of the flow of available carbon dioxide, in particular by regulating a pressure of carbon dioxide gas (CO ₂ gas) at the inlet of the exchanger, which makes it possible to control, regulate the flow of liquid ammonia (liquid NH3) entering the exchanger 6. Thus the expansion valve 22 regulates the flow entering the exchanger 6 during a dry vaporization step. The regulation of the flow section of the expansion valve 22 is here carried out by the control unit 8. Alternatively, any other control member may be provided to regulate the flow section of the expansion valve.

The cooling system 2 further comprises a bypass line 24 which connects the separator pot 20 directly to the outlet of the exchanger 6. In use, steam ammonia (NH 3 vapor) flows from the separator pot 20 in the bypass line 24 to the outlet of the exchanger 6, without passing through the exchanger 6. The bypass line 24 returns the vaporized fraction by free expansion of the medium pressure (here 5 barA) to the low pressure (here 1, 2 barA by process called "flash") to the suction of the compressor 4. In the example of Figures 1 and 2, the separator pot 20 is equipped with a level 29 control member, which is adapted to measure the level of liquid ammonia in the separator pot 20 and to control a physical control element until a predetermined set level is reached.

When the liquid ammonia (liquid NH3) expands from the mean pressure (here 5 barA) to the low pressure (here 1, 2 barA), about 10% of the liquid ammonia goes into the vapor phase. This amount of 10% of ammonia vapor is derived by the bypass line 24. In the example of Figures 1 and 2, the distillation column or the exchanger 6 is equipped (e) a pressure control member 25, which is adapted to measure the carbon dioxide gas pressure and to control a physical control element until a predetermined set pressure is reached.

In addition, the cooling installation 2 comprises a secondary heat exchanger commonly referred to as "subcooler" 23 which is fluidly connected to the exchanger 6 and to the collection tank 10 so as to transfer heat from the carbon dioxide (CO ₂ ) from the exchanger 6 to liquid ammonia (liquid NH ₃ ) from the collection tank 10. In use, the subcooler 23 "consumes" liquid ammonia (liquid NH ₃ ) by contributing to vaporize it . The subcooler 23 operates here according to the principle of thermosiphon. In use, the cooling system 2 operates according to a cooling method according to the invention as shown diagrammatically in FIG.

This cooling process comprises, as shown in FIG. 1, a so-called dry vaporization step 101, in which the exchanger 6 is supplied with gaseous carbon dioxide (CO ₂ gas) and with liquid ammonia (liquid NH ₃ ), and in which all the liquid ammonia (liquid NH ₃ ) leaving the exchanger 6 is vaporized. In this dry vaporization stage, all the ammonia (NH ₃ ) flowing in the exchanger 6 and in the pipe output 1 1 is in the vapor phase.

In practice, vaporizing all the ammonia flowing in the exchanger 6 means that it is necessary to supply the ammonia a quantity of heat which exceeds its enthalpy of vaporization for the quantity of ammonia entered in the exchanger 6.

This cooling method furthermore comprises, as shown in FIG. 2, a supercharging step 102, in which the flow rate of liquid ammonia (liquid NH ₃ ) entering the exchanger 6 is determined so that the ammonia ( NH ₃ ) leaving the exchanger 6 is partially composed of liquid ammonia (liquid NH ₃ ). The supercharging step 102 occurs after the dry vaporization step 101. In other words, the supercharging step 102 is not concomitant with the dry vaporization step 101. In the example of Figure 2, the ammonia (NH ₃ ) leaving the exchanger 6 is partially composed of liquid ammonia (liquid NH ₃ ); therefore the ammonia (NH ₃ ) leaving the exchanger 6 comprises ammonia in the liquid phase and in the vapor phase (NH ₃ liquid + vapor).

This cooling method makes it possible, during the supercharging step 102, to evacuate all or the majority of the oil accumulated in the exchanger 6 during the dry vaporization steps 101. The cooling process may be termed "low temperature" because, in use, the temperature of the ammonia exiting the exchanger 6 is about -30 ° C at a pressure of 1.2 barA.

The cooling process further comprises a cycle step 103 of cyclically and alternately repeating the dry vaporization step 101 and the supercharging step 102. In the cycling step 103, the supercharging step 102 directly follows the dry vaporization step 101. The dry vaporization step has a first predetermined duration for example of about 24 hours and the supercharging step 102 has a second duration predetermined for example about 0.25 h. The ratio of the second predetermined duration (0.25 h) to the first predetermined duration (24 h) is about 1%.

The control unit 8 is configured to control the cooling installation 2 so as to carry out, during the cycle step 1 03:

a first transition from the dry vaporization step 101 to the supercharging step 102 when the low level sensor 12 emits the first signal 10.12, then

a second transition from the supercharging step 102 to the dry vaporization step 101 when the high level sensor 14 emits the second signal 10.14.

In addition, the control unit 8 is configured to control the cooling installation 2 so as to make this first transition so that the flow of liquid ammonia entering the exchanger 6 increases gradually, and then to achieve this. second transition so that the flow of liquid ammonia entering the exchanger 6 decreases gradually.

In addition, the cooling method comprises a control step in which the flow section of the expansion valve 22 is regulated so as to condense the bulk of the flow of available carbon dioxide, in particular by regulating a pressure of the carbon dioxide. gaseous carbon (CO2 gas) at the inlet of the exchanger 6.

In practice, it is possible to regulate the flow rate entering the exchanger 6 during the dry vaporization step 101. In this case, the regulation of the flow section of the expansion valve 22 is carried out by the control unit 8, in particular as a function of the dimensions of the cooling installation 2, in particular of the exchanger 6, and flow rates of ammonia and carbon dioxide.

In addition, the collection tank 10 operates a two-phase separation of the ammonia, so as to separate the liquid ammonia (NH ₃ liquid) from the ammonia vapor (NH ₃ vapor). In addition, the cooling method comprises a channeling step in which the ammonia vapor (NH ₃ vapor) is channeled from the collection tank 10 to the compressor 4, by means of a recirculation pipe 26. In the pipe recirculation 26, upstream of the compressor 4, the ammonia is in the vapor phase (NH ₃ vapor), while downstream of the compressor 4, the ammonia is condensed by an evaporative condenser and is therefore mainly in the liquid phase. The condensed ammonia flows into the storage tank 19, then into the feed tank 18.

The cooling installation 2 further comprises regulating means 28 which are arranged to adjust the heat transferred to the ammonia. In the example of FIG. 1, the regulation means 28 comprise a temperature control member, which is adapted to measure the temperature in the outlet pipe 11 and to control a physical control element, here by opening or closing. of the expansion valve 22 until a predetermined set temperature is reached.

During the dry vaporization step 101, the regulation means 28 are regulated so as to superheat the ammonia leaving the exchanger 6. Advantageously, the control unit 8 regulates the regulation means 28 so that the temperature of the ammonia leaving the exchanger 6 is at least 3 ° C. higher than the dew point temperature of the ammonia, calculated at the operating pressure, in this case 1.2 barA.

In use, when the high level sensor emits the second signal (10.14), the regulating means 28 supply the ammonia with a quantity of heat which exceeds the evaporation enthalpy for the amount of ammonia entering the exchanger 6 , which makes it possible to carry out the dry vaporization step 101.

During the booster stage 102, the flow rate of liquid ammonia entering the exchanger 6 is determined so that the ammonia leaving the exchanger 6 comprises a mass of liquid ammonia representing more than 10% of the mass of ammonia vapor.

In the example of FIGS. 1 and 2, the separator pot 20 is equipped with a level control member 29, which is adapted to measure the level of liquid ammonia and to control a physical control element until it reaches a desired level. predetermined set level which is known to be high enough to create an excess of liquid ammonia at the outlet of the exchanger 6 during the supercharging step 102.

Of course, the invention is not limited to the particular examples described in the present application. Other embodiments or variants within the scope of the art can also be envisaged without departing from the scope of the invention defined by the claims below. For example, FIG. 4 illustrates a variant of the invention in the supercharging step 102, in which the cooling installation 2 does not comprise a separator pot, unlike the cooling installation 2 illustrated in FIGS. and 2.

Claims

1. Cooling method for cooling a heat transfer fluid by heat exchange with a refrigerant, for example for condensing gaseous carbon dioxide by heat exchange with ammonia, the cooling process using a cooling system (2) comprising at least one heat exchanger (6) adapted to transfer heat from the circulating fluid to the refrigerant, the cooling system (2) comprising a compressor (4) lubricated and adapted to compress refrigerant,

the method of cooling comprising a so-called dry vaporization step (101) including actions of: i) supplying the exchanger (6) with circulating fluid and liquid refrigerant, and ii) vaporizing all the liquid refrigerant s' flowing in the exchanger (6),

the cooling method being characterized in that it further comprises a supercharging step (102) which occurs after the dry vaporization step (101) and which includes actions of: i) feeding the exchanger ( 6) in circulating fluid and liquid refrigerant, and ii) adjust the flow rate of the liquid refrigerant entering the exchanger (6) so that the refrigerant leaving the exchanger (6) comprises liquid refrigerant.

The cooling method according to claim 1, further comprising a cycle step (103) of cyclically repeating the dry vaporization step (101) and the supercharging step (102), the vaporizing step of sec (101) having a first predetermined duration and the boosting step (102) having a second predetermined duration, a ratio of the second predetermined duration to the first predetermined duration being preferably less than 10%.

Cooling method according to claim 2, wherein the cooling system (2) further comprises: a collecting tank (1 0) arranged to contain liquid refrigerant leaving the exchanger (6) and the gaseous refrigerant leaving the exchanger (6),

a first sensor, said low level sensor (1 2), adapted to emit a first signal when the volume of liquid refrigerant contained in the collection tank (1 0) reaches a predetermined lower threshold (1 0.1 2),

a second sensor, said high level sensor (14), adapted to emit a second signal when the volume of liquid refrigerant contained in the collection tank (10) reaches a predetermined upper threshold (10.14), and - a control unit (8) configured to control the cooling installation (2) to realize:

a first transition from the dry vaporization step (101) to the supercharging step (102) when the low level sensor (12) transmits the first signal, and then

a second transition from the supercharging step (102) to the dry vaporization step (101) when the high level sensor (14) transmits the second signal.

The cooling method according to claim 3, wherein the control unit (8) is configured to control the cooling installation.

(2) so as to realize the first transition so that the flow rate of the liquid refrigerant entering the exchanger (6) increases gradually, and then to achieve the second transition so that the flow of the liquid refrigerant entering the exchanger (6) decreases gradually.

Cooling method according to any one of claims 3 to 4, wherein the cooling system (2) further comprises:

a supply tank (1 8) adapted to contain liquid refrigerant under a so-called mean pressure, for example 5 barA,

- a separator pot (20) adapted to separate the liquid refrigerant from the gaseous refrigerant under a so-called low pressure, for example 1.2 barA, which is lower than the mean pressure, the separator pot (20) being arranged between the supply tank (18) and the exchanger (6) so as to receive refrigerant of the supply tank (18) and to transmit the liquid refrigerant preferably to the exchanger (6) by gravity,

an expansion valve (22) arranged between the supply tank (18) and an inlet of the exchanger (6) so as to relax the liquid refrigerant from the mean pressure to the low pressure,

a collection tank (10) arranged to contain liquid refrigerant leaving the exchanger (6) and the gaseous refrigerant leaving the exchanger (6), so as to recover the oil entrained at the outlet of the exchanger (6) and to separate any excess liquid refrigerant,

the cooling method further comprising a control step in which the flow section of the expansion valve (22) is regulated so as to condense the bulk of the flow of available heat transfer fluid, in particular by regulating a caloric fluid pressure to the inlet of the exchanger (6), and thus to control the flow of liquid refrigerant entering the exchanger (6).

Cooling method according to any one of claims 3 to 5, wherein the collection tank (10) operates a two-phase separation of the refrigerant so as to separate the liquid refrigerant from the gaseous refrigerant, the cooling process comprising in addition a channeling step in which the gaseous refrigerant is channeled from the collection tank (10) to the compressor (4).

Cooling method according to one of the preceding claims, wherein the cooling system (2) further comprises regulating means (28) for transferring heat to the refrigerant so as to superheat the refrigerant. leaving the exchanger (6),

preferably by regulating the flow rate of the refrigerant and the flow rate of the circulating fluid so that the temperature of the refrigerant leaving the exchanger (6) is at least 3 ° C higher than the dew point temperature of the refrigerant , calculated at the operating pressure, for example 1.2 barA.

Cooling method according to one of the preceding claims, wherein the refrigerant is selected from the group consisting of ammonia, freon, ethane, propane and any mixture of said refrigerants, and the circulating fluid is selected from the group consisting of carbon dioxide, nitrous oxide, hydrocarbons, in particular hydrocarbons whose molecule comprises two to four carbon atoms, or any mixture of said caloric fluids, with possibly impurities such as nitrogen, oxygen, hydrogen, methane and / or carbon monoxide.

9. Cooling method according to claims 7 and 8, wherein the refrigerant is ammonia, the circulating fluid being for example carbon dioxide pure or impure, and wherein the temperature of the refrigerant leaving the exchanger (6) is between -20 ° C and -50 ° C, for example about -30 ° C.

Cooling method according to one of the preceding claims, wherein, during the supercharging step (102), the flow rate of the liquid refrigerant entering the exchanger (6) is determined so that the fluid refrigerant leaving the exchanger (6) comprises a mass of liquid refrigerant representing at least 10% of the mass of gaseous refrigerant.

1 1. Cooling plant (2) for cooling a circulating fluid by heat exchange with a refrigerant, for example for condensing gaseous carbon dioxide by heat exchange with ammonia, the cooling system (2) comprising: at least one exchanger (6) adapted to transfer heat from the circulating fluid to the refrigerant, the cooling system (2) comprising a compressor (4) adapted to compress refrigerant,

the cooling system (2) being characterized in that it further comprises a control unit (8) configured to operate the cooling system (2) according to:

a so-called dry vaporization step (101) including actions consisting in: i) supplying the exchanger (6) with circulating fluid and fluid liquid refrigerant, and ii) vaporize all the refrigerant flowing in the exchanger (6), and

a supercharging step (102) that occurs after the dry vaporization step (101) and includes actions of: i) supplying the heat exchanger (6) with circulating fluid and liquid refrigerant, and ii) adjusting the flow rate of the liquid refrigerant entering the exchanger (6) is such that the refrigerant leaving the exchanger (6) is composed wholly or partially of liquid refrigerant.

12. Cooling plant (2) according to claim 1 1, wherein the exchanger (6) is a plate heat exchanger, preferably brazed aluminum.

13. Cooling plant (2) according to any one of claims 1 1 to 12, further comprising:

a collection tank (1 0) arranged to contain liquid refrigerant leaving the exchanger (6) and the gaseous refrigerant leaving the exchanger (6), so as to recover the oil entrained at the outlet of the exchanger (6) and to separate any excess liquid refrigerant,

a first sensor, said low level sensor (1 2), adapted to emit a first signal when the volume of liquid refrigerant contained in the collection tank (1 0) reaches a predetermined lower threshold (1 0.1 2), and

a second sensor, said high level sensor (14), adapted to emit a second signal when the volume of liquid refrigerant contained in the collection tank (10) reaches a predetermined upper threshold (10.14).

14. Cooling plant (2) according to claim 1 3, further comprising a secondary exchanger (23) fluidly connected to the exchanger (6) and the collection tank (1 0) so as to transfer heat from the fluid heat exchanger from the exchanger (6) to the liquid refrigerant from the collection tank (10).

15. Cooling plant (2) according to any one of claims 13 to 14, further comprising:

a separator pot (20) adapted to separate the liquid refrigerant from the gaseous refrigerant under a so-called low pressure, for example of 1.2 barA, which is lower than the mean pressure, the separator pot (20) connecting the storage tank; supply (18) to the exchanger (6) so as to receive refrigerant from the supply tank (18) and to supply the exchanger (6) with liquid refrigerant preferably by gravity, the volume of the separator pot ( 20) being smaller than the volume of the feed tank (1 8),

an expansion valve (22) arranged between the supply tank (18) and an inlet of the exchanger (6) so as to relax the liquid refrigerant from the medium pressure to the low pressure and thus to control the flow rate of liquid refrigerant entering the exchanger (6).

16. Cooling plant (2) according to claim 15, further comprising a bypass line (24) connecting the separator pot (20) directly to the outlet of the exchanger (6).